1. Field of the Invention
This invention relates to a highly heat-resistant glass fiber for use in optical transmission (hereinafter referred to as an optical fiber) and to a process for producing the same.
2. Description of the Prior Art
With the recent advancement of fiber optics technology on reducing light loss due to absorption by an optical fiber, many attempts have been made to use the optical fiber as a medium for communication, and the day is not far off when commercial application of optical transmission will be achieved. While optical fibers housed in a cable (hereunder referred to as an optical cable) can be used for a variety of communication modes, the electrical noninduction of the optical fiber has interested researchers in the possibility of combining the optical cable with a power cable in a composite communication power cable. Therefore, various methods have been proposed for housing optical fibers within a power cable and using them to transmit control or communication signals. In addition, some methods are being subjected to field testing.
It is known that the conductor and neighboring area of a power cable, especially a high-tension cable, are heated to about 90.degree. C. by the Joule effect. Therefore, it is necessary to ensure that an optical fiber exposed to this temperature level for a long period of time will not undergo any change in its transmission or mechanical characteristics.
An optical fiber is generally protected with a plastic coating to increase its mechanical strength or to make its handling easier. Accompanying FIGS. 1 to 3 illustrate the coating of optical fibers proposed to date. Therein 1 is a glass fiber, 2 is a primary coating layer of a thermoplastic resin, 3 is a primary coating layer of a thermosetting resin, and 4 is a buffering layer made of a resin with small Young's modulus or a foamed resin. According to the study of the present inventors, if the coating of the optical fiber (hereunder referred to simply as an optical fiber) is exposed to high temperatures, shrinkage due to the residual strain carried over from molding of plastics or a change in the volume accompanying higher crystallinity will increase the "clamping" effect on the fiber interior to cause a phenomenon generally referred to as "micro-bending", which in turn results in greater transmission loss.
With the above point in mind, research was carried out to provide a highly heat-resistant optical fiber and an optical cable housing such fiber. Thus the present invention provides a glass fiber for use in optical transmission which comprises a first layer containing an organopolysiloxane having a refractive index higher than that of the glass which forms the outermost layer of the glass fiber, a second layer comprising an organopolysiloxane which is the same or different than the organopolysiloxane in the first layer, and a thermoplastic resin coated on the second layer in a thickness less than about 100 microns.